Many available transducers, especially microminiaturized transducers, use piezoresistive deformation sensing. Piezoresistive sensing is very effective because it is nearly insensitive to lay-out dependent parasitics, which is not the case with capacitive sensing, and because resistive sensing provides linear output signals which can be temperature compensated. The primary problems for piezoresistive sensing are in data extraction. The change in resistance over the deformation span of a pressure transducer is typically 1% or so, which means that precision DC-amplifiers are required, and major problems can occur due to device imperfections which would not be as troublesome if the output signals were larger.
Mechanically resonant structures have been utilized as force transducers. In such transducers, the applied force causes a change in the resonant frequency of the resonating structure, e.g., a resonating wire or beam. Resonating transducers have been demonstrated which have a change of frequency of more than 100% caused by forces which are much smaller than those which will cause a 1% change in the resistance of a piezoresistive sensor. The transduction sensitivity for resonating transducers is therefore much higher than for piezoresistive transducers. Because resonant transducers use data extraction circuitry which measures frequency or time, the amplitude of the signal from the transducer is not crucial. In addition, because frequency or time is being measured, the output of the transducer is easily converted to a digital output signal, which is well adapted for use by digital processing systems.
Although mechanically resonant transducers are typically very expensive, their expense is balanced against the advantages mentioned above, as well as their extreme accuracy and insensitivity to temperature. However, because such resonating structures are highly sensitive to forces, even small temperature changes can affect performance in packaging structures for the resonator which are formed from thermally mismatched materials. A well designed resonating transducer will avoid such thermal mismatch by utilizing a single material for the resonator and the mechanical structure with which it is incorporated. The cost of such transducers can be minimized if they can be made by bulk manufacturing techniques.
An all silicon resonating beam transducer has recently been developed by Yokogawa Electric Corporation. See K. Ikeda, et al., "Silicon Pressure Sensor with Resonant Strain Gauges Built Into Diaphragm," Technical Digest of the 7th Sensor Symposium, Shigaku Kaikan, Tokyo, Japan, 30-31 May 1988, pp. 55-58, and K. Ikeda, et al., "Three-Dimensional Micromachining of Silicon Resonant Strain Gauge," id., pp. 193-196. A related device is shown in U. K. Published Patent Application GB 2,180,391A, Application No. 8620782, published Apr. 1, 1987, entitled "Vibratory Transducer and Method of Making the Same". The Yokogawa device is manufactured by using selective epitaxy and preferential chemical etches. The device is made from silicon with several different doping levels. The resonator is an H-type flexure resonator with four clamped supports at the end points of the H. Because the manufacturing process uses a P++ etch stop (heavily boron doped silicon which does not etch in, for example, hydrazine) the conductivity of the H resonator is very large. The high conductivity of the resonator makes an H structure preferable to a simple beam, and may reduce the available choices for sensing and electronic excitation. The foregoing papers report excitation of the resonator utilizing a magnetic bias field supplied by a permanent magnet along the plane of the wafer, with current forced through one of the beams of the H. This results in a vertical force. The second beam of the H is slaved to the motion of the driven beam of the H via the interior or joining beam of the H. An output voltage, which is magnetically induced, is sensed on the second beam. This output signal is amplified and fed back to the driven beam to maintain the oscillation at resonance. The fundamental mode resonant frequency for the device is on the order of magnitude of 50 kilohertz (50 KHz) or less. The strain field in the beam material ordinarily will be highly tensile in such structures and any process variations may cause such devices to exhibit variations in resonant frequency. The heavily doped silicon ordinarily exhibits a large amount of mechanical slip, which can affect the strength and long term stability of the resonator.